MUSEUM OF COMPARATIVE ZOOLOGY. 113 



of these nuclei affords no clue as to which oue belongs to the rudi- 

 mentary cell. 



In Palinurus (Plate X. Fig. 125, nl. px.), the eighth nucleus is regu- 

 larly present and easily seen. In Cancer (Fig. 129, nl. px. 8) it occu- 

 pies a position between the adjacent retiuulae. It can also be identified 

 in Crangon. 



The retinula? in Decapods, according to all recent observers, contain 

 seven functional cells. In Homarus, Palinurus, Cambarus, Crangon, 

 Pakemonetes, and Cancer, the retinulae contain, in addition to the 

 seven nuclei of the functional cells, an eighth nucleus, which repre- 

 sents, I believe, a rudimentary cell. It is probable, therefore, that in 

 all Decapods each retinula really contains eight cells, one of which is 



rudimentary. 



The rhabdome in Decapods presents a very uniform structure. It is 

 usually an elongated body, pointed both at its distal and its proximal end, 

 and completely covered, except at its distal tip, by the proximal retinular 

 cells. In those Decapods in which it is large enough to be conveniently 

 observed, its transverse section is squarish, and usually subdivided by 

 two straight lines into four smaller squares (Plate IX. Fig. 113). As 

 Grenacher ('77, pp. 31, 32) first demonstrated in Palaemon, the retinular 

 cells are rather peculiarly arranged around the rhabdome. One of its 

 four sides is flanked by one cell, the other three by two cells each. This 

 arrangement can be seen in Palsemonetes (Fig. 113), and probably obtains 

 for all Decapods. 



In Palinurus Argus (Plate X. Fig. 124) there appears to be no rhab- 

 dome, unless the translucent axial portion of each retinular cell can 

 be said to represent segments of it. The fibrous ends of the cone cells 

 (cl. con.) can be easily identified between the retinular cells, but the 

 centre of the retinula is filled with pigment, and shows not the least trace 

 of a rhabdome. This peculiarity of Palinurus was noticed as early as 

 1840 by Will ('40, p. 15), who described the ommatidium in this genus 

 as being without a transparent mass (= rhabdome). 



Although the distal retinular cells in Decapods were collectively rec- 

 ognized by Midler ('26, pp. 355, 356) some sixty years ago as a definite 

 pigment band in the distal portion of the retina in the crayfish, they 

 were not identified as separate cells until quite recently. The first in- 

 vestigator to observe them was Carriere ('85, p. 169), who described 

 them in Astacus as a pair of pigment cells flanking each cone. In Cam- 

 barus, Crangon, and Homarus, they also cover the sides of the cone, and 

 in the last named genus they are produced proximally into long fibres, 



vol. xxi. — no. 2. 8 



